The present invention relates to solar cells and, more particularly, to thin film solar cells and methods for making the same.
Thin film solar cells are relatively inexpensive to produce and, thus, are widely utilized. Due to its low production cost, the energy efficiency of thin film solar cells can be continuously developed and improved. One objective in developing cell efficiency involves making a p-type window layer electrically conductive, while providing the film with low resistivity, good light transmission in every wavelength and low light absorption coefficient. At present, the most widely used thin film is p-type amorphous silicon carbide (p-type a-SiC) fabricated using a plasma enhanced chemical vapor deposition (PECVD) process, which uses Silane gas (SiH4), Diborane gas (B2H6), Methane gas (CH4) and/or Hydrogen gas (H2).
Cell efficiency may be improved by developing a thin film solar cell having a lower electrical resistance. Low electrical resistance of the film, leads to low resistance of solar cell and better electrical conductivity from the solar cell to the substrate materials. In addition, the thin film solar cell should include a good window layer having a low light absorption coefficient. In the past, p-type microcrystalline silicon carbide thin film was produced, utilizing an electron cyclotron resonance-chemical vapor deposition (ECR-CVD) process using high electromagnetic frequency of 2.4 GHz. By this method, p-type amorphous silicon carbide thin film has also been formed. The resulting thin film has a good window layer. However, in the production of films having a large area, this process may result in poor uniformity of the p-type layer. Accordingly, production of effective and efficient large scale solar cells for commercial use may be difficult.
The problem of forming p-type amorphous silicon carbide thin film over a large area has led to the development and proposal of producing p-type microcrystalline silicon oxide (p-type μc-SiO) thin film through PECVD method using a radio frequency of 13.56 MHz. See U.S. Pat. No. 5,507,881 to Sichanugrist et al. Later, amorphous silicon thin film solar cell was applied on substrate materials, such as stainless steel. Despite these developments, p-type microcrystalline silicon oxide thin film may be formed on certain substrate materials of large size, such as stainless steel, but have not yet be formed on glass substrate. Up to now, p-type amorphous silicon oxide thin film has been used in the production of solar cell on glass substrate.
Accordingly, a need remains for a p-type microcrystalline silicon oxide thin film solar cell on glass having a high efficiency and low production cost.